[unreadable] [unreadable] Pregnancy has been shown to have both a short- and long-term effect on breast cancer risk; a short-term (3-5 year) increase in breast cancer risk is followed by a long-term (> 10 year) reduction in risk and both associations appear to be modified by age at delivery. Hormonal factors have also long been implicated in the etiology of breast cancer. Though exact biological mechanisms are unknown, the short-term increase in breast cancer risk associated with pregnancy has been hypothesized to be attributable to the substantially elevated levels of multiple hormones (e.g., estrogens, progesterone, human chorionic gonadotropin (HCG), and insulin- like growth factor (IGF-1) seen during pregnancy. Offspring birth characteristics (e.g., preterm delivery or low or high fetal growth) may well be associated with an altered hormonal environment during pregnancy and influence maternal breast cancer risk, but associations between offspring birth characteristics and breast cancer risk are not well established. The overall objective of this study is to examine the associations between offspring birth characteristics that have inconsistently been implicated in breast cancer risk and premenopausal breast cancer (e.g., 50 years of age or less). Specifically, we will examine the association between preterm (< 37 weeks gestation) and very preterm delivery (< 32 weeks gestation), fetal growth (birthweight-for-gestational age), multiple births and breast cancer risk. Since age at first birth and subsequent deliveries, and time since birth, have been shown to influence the effect of pregnancy on maternal breast cancer risk, we will also examine potential modification of associations between offspring birth characteristics and breast cancer risk by age at first delivery (= 30, > 30 years), age at delivery of each subsequent pregnancy (= 30, > 30), and time since delivery at breast cancer diagnosis (< 5 years, = 5 years). Breast cancer incidence has also been shown to vary by race and socioeconomic status; since these factors may also modify our exposures of interest, we will examine potential effect modification of associations between offspring birth characteristics and breast cancer risk by race (Non-Hispanic White, Non-Hispanic Black), and socioeconomic status (based on age- appropriate education level at first delivery). We propose to conduct a registry-linked, population-based, case- control study using Michigan maternally-linked birth certificate data (1978-2003) and the Michigan state-wide cancer registry data (1985-2005) among parous Non-Hispanic Black and Non-Hispanic White women 50 years of age or less to examine these associations. Results from this study of the association between birth characteristics that may reflect an altered maternal hormonal milieu during pregnancy and breast cancer, may provide insight into potential biological mechanisms for the role of pregnancy in premenopausal breast cancer risk. Given the heterogeneity of our study population, and ample sample size, results may also shed light on observed differences in incidence rates of premenopausal breast cancer among Black and White women and women of different socioeconomic status. [unreadable] [unreadable] [unreadable] [unreadable]